The present invention relates to construction techniques for reflection holograms, and in particular, an apparatus and a method for constructing reflection holograms of selectable equivalent thickness that are free from the effects of parasitic holograms that cause multiple images to appear in optical display systems incorporating them.
It is known that during reflection hologram construction, flare-causing parasitic transmission holograms develop as a consequence of multiple reflections of light energy in the hologram recording apparatus from the surfaces which form interfaces of materials of different indices of refraction. The reflected light energy interacts with the construction light rays to form the parasitic holograms. Solutions for suppressing the formation of parasitic holograms in diffusion holograms, reflection holograms with a single beam exposure system, and reflection holograms with a double beam exposure system are described in the respective U.S. Pat. Nos. 4,456,328; 4,458,977; and 4,458,978 of Arns et al., all of which are assigned to Hughes Aircraft Company. Only U.S. Pat. No. 4,458,977 ('977 patent) merits discussion.
The '977 patent describes a hologram recording structure that includes a single point source from which a first construction beam of light rays emanates and passes through a holographic recording material applied on a curved surface. More specifically, the light rays pass through a cover plate, a first index matching fluid layer, the recording material, a second index matching fluid layer, and then reflect from a curved mirror to form a second construction beam of light rays that interfere with the incident first construction beam of light rays to form the hologram. The recording structure has a mechanism which moves the mirror and the recording material together with respect to the cover plate to continuously change the phase of undesired light energy reflected from the cover plate and thereby prevent the formation of spurious holograms.
In FIG. 5, and column 7, lines 45-53, the '977 patent describes the use of a global phase shifter that is positioned between the point source and the cover plate. The global phase shifter corrects the phase of the first construction beam to maintain a constant phase for each of its light rays so that at any point on the recording material the first and second construction beams maintain their relative phase relationship.
The recording structure of the '977 patent suffers from the disadvantage of requiring a very complicated mechanism for changing the phases of the reflected light energy relative to the construction beams to prevent the formation of spurious holograms. Moreover, the '977 patent does not contemplate the effect of the position of the source and the mirror in forming a pattern of holographic isoindex fringe surfaces that intersect the surface of the substrate, thereby creating a weak transmission grating effect, which is another source of flare.
A technique for reducing flare resulting from the transmission grating effect is disclosed in U.S. Pat. No. 4,582,389 of Robert B. Wood and Robert F. Cannata, for "Holographic Device." The method of making the hologram described in the '977 patent is incompatible for use with the technique described in the patent of Wood et al. because one of the reflected light ray components which the '977 patent seeks to suppress is that upon which the patent of Wood et al. relies to form the primary hologram.
U.K. Patent Application GB 2 071 866 A of The Marconi Company Limited describes a method of making a holographic reflector. The method includes the steps of directing a laser beam toward a photosensitive film and repeatedly illuminating on the film adjacent beam spots of a very small area as compared with the total area of the film. A mirror positioned adjacent to the film reflects the laser beam to provide a second construction beam. The laser beam is moved relative to the film so that each point on the film is illuminated numerous times so that the total illumination received by each point is substantially constant. The beam is scanned systematically over the surface of the film in a sequence of line scans with adjacent line scans being spaced apart by a distance that is small compared with any dimension of the beam spot on the film.
Exposing the photographic film in this manner would apparently prevent the formation of parasitic holograms because the reflected light energy would not interfere with the light rays of the construction beams which illuminate a very small area on the photographic film at any given instant during exposure. The recording method of The Marconi Company suffers from the disadvantage of requiring an exposure time for each spot on the photographic film of about 10 seconds. The total time required to complete construction of the hologram would be, therefore, prohibitively long.